We just had a new mattress made for our trailer- we had Dormir
(local Calgary company) make it for us. It cost $300 including a new cover for a 6" 2lb foam mattress. (I think the size was 45" x 75".) I am also around 220 lbs, and it is a HUGE improvement over the mattress the trailer came with.
When I was shopping around I found prices anywhere from $150 up to $600. The big difference was the $150 ones felt cheap; and I have learned if it feels cheap it won't last. Going up to the $500-600 range was just getting silly with the multiple layers of foam. May have been worth it if that was our every day bed. Once again- finding that line between making camping a little more comfortable, and going overboard.